In a paper published in Marine Ecology, Michael J. Sweet of the UK's University of Newcastle writes that "reef corals can adapt or acclimatize to changing environmental conditions in three principal ways: (i) with direct changes in gene expression and the physiological traits of the coral and/or its symbiotic algae (Brown et al., 2000, 2002), (ii) adaptation by replacement of susceptible symbiotic algae with genetically distinct, more resistant symbionts acquired from the environment (Baker et al., 2004; Rowan, 2004; LaJeunesse et al., 2010), or (iii) 'symbiont shuffling' whereby shifts in the existing populations occur such that the dominant, more susceptible symbionts are replaced by rarer, more resistant types (Baker et al., 2004; Berkelmans and van Oppen, 2006)." And he also notes that "to date, approximately 85% of cnidarian host species have been shown to acquire their Symbiodinium complement from the environment at some state in their life history," citing Harrison and Wallace (1990).

Delving deeper into the subject in a study conducted at Heron Island on the Great Barrier Reef of Australia during March 2009, when sea surface temperatures (SSTs) were relatively warm, and during August 2010, when SSTs were cooler, Sweet examined spatial changes in Symbiodinium clades within the water column and compared them with those found in other environmental pools, such as biofilms, sediments, and mucus mats, as well as those found within the study site's dominant reef-building species (Acropora muricata). And what did he thereby learn?

Sweet detected a diversity of algal symbiont clades in the several environmental pools that he studied, "with specific clades associated with different habitats," noting that "at an island scale, there was significant variation in clade composition between sites separated by 0.5-0.7 km," a result that was repeated for both sample periods (March 2009 and August 2010). And so he goes on to conclude that (1) "although no single environmental pool contained a Symbiodinium complement comparable to that of the host coral species investigated, the dominant coral Symbiodinium were available in combinations of the environmental pools, indicating that the coral has the potential to obtain its symbionts from a variety of environmental sources," and that (2) "should environmental conditions select against the current endosymbiotic complement, individuals in the environmental assemblage may be capable of exploiting the newly available 'endosymbiotic niche'." And that is great news for those worried about the future status of Earth's corals!

Berkelmans, R. and van Oppen, M.J.H. 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. Proceedings of the Royal Society B: Biological Sciences273: 2305-2312.